Microwave applications have been found in fields ranging from wireless communications, radar technology navigation, radio-astronomy, imaging, etc. Often, these applications operate with a high data rate or in high resolution. In view of these large uses of microwave applications, there is a trend in the industry to use the working frequencies of the microwave ranges to millimeter-wave ranges in various systems.
In the exploring of circuits in millimeter wave bands, the transmission line of millimeter-wave bands is an important part of the design and application of millimeter-wave technology. This is because a transmission line is the basic element for building passive/active components. However, conventional transmission lines using printed circuit technology such as microstrip lines and coplanar waveguides which have been used in microwave hybrid and monolithic integrated circuits operate poorly in practice. This is due to the fact that these lines and waveguides fail to meet low-loss requirement at the millimeter-wave ranges, partially, due to the serious losses of the millimeter-wave signal through the transmission lines.